This invention relates to a solid-state imaging device for use in a television camera etc., and more particularly to the structure of a picture element thereof.
FIG. 1 is a schematic diagram showing the arrangement of a solid-state imaging device. A photoelectric conversion element made of a photodiode 1, and an insulated-gate field effect transistor (hereinbelow, abbreviated to "MOS transistor") 2 for a vertical scanning switch constitute one picture element. Using a horizontal scanning circuit 9 as well as a vertical scanning circuit 10 made of a shift register and MOS transistors 3 for horizontal scanning switches, optical signals stored in the photodiodes 1 are sequentially led out to an output terminal 8 in conformity with the scanning type of television. The figure illustrates the basic form. Especially regarding the signal output system, there are various modifications, for example, the provision of a plurality of signal output lines 6 or 7 for simultaneously delivering color signals in color imaging systems, and the construction of scanning circuits employing charge transfer devices. In addition, there is a modified vertical scanning system conforming with the interlaced scanning of television. Further, there is a modification pertaining to picture elements for improved photoelectric conversion characteristics as disclosed in U.S. Pat. No. 4,148,048. The ensuing description is similarly applicable to these modifications. While devices whose signal charges are electrons will be described, the ensuing description is quite similarly applicable to devices utilizing holes as their signal charges, merely by inverting the polarity and the conductivity types.
FIG. 2 is a schematic sectional view of a picture element. The problems of the prior art will be explained with reference to this figure.
A p-type Si substrate 12 and an n-type diffusion layer 13 construct the photodiode 1, while the n-type diffusion layer 13, a gate electrode 15 made of polycrystalline Si or the like and an n-type diffusion layer 14 serving as an output terminal construct the switching MOS transistor 2. A photoelectron 19 generated by the incidence of light 18 is stored in a junction capacitance 20, and a positive voltage pulse issued by the vertical scanning circuit 10 is impressed on the gate electrode 15, whereby the photoelectron is delivered through the n-type diffusion layer 14 every scanning period. Numeral 16 designates an insulating film made of SiO.sub.2 or the like.
This picture element of the prior art has serious disadvantages in the following two points.
The first concerns noise charges. Numeral 23 represents photoelectrons generated within the Si substrate 12 which form a component which does not flow into the n-type diffusion layer 13 but flows instead into the n-type diffusion layer 14, as shown by arrow 24. This component is superposed on the signal of another picture element sharing the vertical signal output line 6 (a metal interconnection 17 in FIG. 2) when the signal is read out. In case of imaging a bright object, therefore, a bright line or stripe appears in the vertical direction in a reproduced picture and spoils the picture quality (this phenomenon is called a "smear"). This is a serious disadvantage which is common to solid-state imaging devices and on account of which they are much inferior to image pickup tubes.
The other disadvantage concerns parasitic capacitances. The n-type diffusion layer 14 has a junction capacitance 21 similar to the junction capacitance 20 of the photodiode. The vertical signal output line 6 connects such junction capacitances 21 in parallel to the number of picture elements arrayed in the vertical direction (this number depends upon the signal readout system, and is about 250 or 500 in case of the NTSC standard format). The resultant parasitic capacitance 21 becomes very large. The parasitic capacitance causes the occurrence of thermal noise, in which a small photoelectric conversion signal becomes buried. In effect, the sensitivity of the imaging device is thereby drastically lowered.